Diatomaceous Earth. Coloured scanning electron micrograph (SEM) of diatomaceous earth consisting of fossilized diatoms. This sample is from Loch Cuithir on the Isle of Skye, Scotland. Diatoms are single-celled planktonic algae. The diatoms create a shell (frustule) of silica around themselves for protection and support which is readily fossilised. Rock formed from diatom shells is called diatomite or diatomaceous earth. It is used industrially as a filter for purifying liquids. Magnification x290 at 10cm wide.